*
 * Keys are stored wrapped in {@code WeakIdentityHashMap.WeakKey} weak reference implementation, that uses {@code System.identityHashCode}
 * for generating hash code and considers referent equality for {@code equals} method.
 */
class WeakIdentityHashMap<K, V> {
    private final HashMap<WeakKey<K>, V> map = new HashMap<>();

    void put(K key, V value) {

        WeakIdentityHashMap.WeakKey<Thing> weakKey1 = new WeakIdentityHashMap.WeakKey<>(thing1)
        WeakIdentityHashMap.WeakKey<Thing> weakKey2 = new WeakIdentityHashMap.WeakKey<>(thing1)

        expect:
        weakKey1 == weakKey2
    }

    def "hashCode of weakKey is system identity hashCode"() {
        Thing thing = new Thing("thing")